Viability of Using DGT Passive Samplers to Measure Dissolved Trace Elements in Subtropical Freshwater
and Estuarine Environments
Master of ScienceThesis Defense by
Michael S. Tomlinson
Acknowledgments (mahalo nui loa!)➟ Thesis Committee:
➟ Eric De Carlo (Chair)➟ Fred Mackenzie➟ David Karl➟ Khalil Spencer
➟ The Team:➟ Scott Narod➟ Vincent Todd➟ Norine Yeung➟ Sam Saylor➟ Vincent Beltran
➟ Others:➟ Dan Hoover➟ Hao Zhang (Lancaster Univ.)➟ Nancy Koike➟ Kathy Kozuma
➟ Funding/Other Support:➟ NOAA Sea Grant➟ USEPA➟ Hawai�i DOH & DLNR➟ NSF (ICP-MS)➟ USGS
➟ To quantify dissolved trace element inputs to aquatic habitats:➟ Methods time consuming and expensive➟ Ambiguous, definitions of dissolved vary➟ Discrete water samples are snapshots in time➟ Relation between sediment/tissue
concentrations difficult to relate to water column concentrations
➟ Often trace element concentrations <MDL
Motivation for the study
Nonpoint source pollution (after NPDES)➟ �Nonpoint source [NPS] pollution . . .
a significant factor in coastal water degradation� (U.S. Congress, 1990)
➟ �Stormwater linked to major coastal problems� (EPA, 1993)
➟ �May be greatest threat to marine ecosystems� (Clark, 1995)
➟ bioavailability can ultimately threaten human health through consumption of aquatic organisms
Why dissolved trace elements?
➟ Dissolved phases considered bioavailable
➟ �Bioavailability�the fraction of total contaminant in surrounding medium which is correlated with a quantitative biological response such as biomagnification� (EPA, 1992)
➟ Definition of �dissolved� is operational & varies with filter pore size (typically 0.2 to 1 µm)
Measuring NPS pollution➟ Water column sampling
➟ Sediment sampling
➟ Bioaccumulation in resident & caged species (e.g., NS&T Mussel Watch)
➟ Passive samplers
Water column sampling:➟ Concentrations may be <MDL➟ Snapshot in time➟ Sampling, containment, &
preservation can alter chemistry
➟ Filtering can alter chemistry➟ Ambiguity between dissolved
& particulate phases
Sediment sampling:➟ Sediments tend to be patchy
requiring numerous replicates➟ Bioturbation & other disturbances
can confound results➟ Difficult to obtain undisturbed
sediment sample➟ Sampling, containment, &
preservation can alter chemistry➟ Concentration relation
[sediment] ≠ [water column]
Organism bioaccumulation:➟ Difficult to locate suitable type/quantity of animals➟ May accumulate dissolved & particulate pollutants➟ Inter- & intra-specific comparisons difficult➟ Animals can metabolize or depurate pollutants➟ Non-sessile organisms can move in & out of area➟ Concentration relation [organism] ≠ [water column]
NOAA NS&T Mussel Watch Program
Ostrea sandvicensis (Hawai�i)Mytilus edulis (Maine to Delaware Bay &
US West Coast)
Passive samplers:➟ Relatively recent development➟ Time integrating device➟ Accumulate bioavailable pollutants
(exclusive of ingestable particulate matter & larger colloids)
➟ Measure pollutants <MDL➟ Relatively inexpensive➟ SPMDs sample dissolved nonpolar
organics (e.g., PCBs, pesticides)➟ DGTs sample dissolved trace elements
DGT
SPMD
DGT (Diffusive Gradients in Thin-films)➟ Developed by Davison and Zhang (1994) of
Lancaster University➟ Measures dissolved Cd, Cr, Cu, Pb, Zn, Co, Ni,
Ag, Mn, Fe, Al➟ Work in saltwater, freshwater, sediments & soils➟ Consists of membrane filter, diffusive hydrogel,
resin gel, and housing (see diagram)➟ Effective pore size generally 0.002-0.005 µm
& no >0.020 µm (�standard� DGT)➟ Inexpensive (£10 or ~$17, March 2002)
Components of a DGT sampler➟ outer sleeve & piston➟ 0.45-µm, polysulfone membrane filter➟ polyacrylamide hydrogel (~95% water)➟ layer of Chelex-100® resin in hydrogel
Cb = bulk solution concentrationDBL = diffusive boundary layerδ = DBL thickness∆g = diffusive gel thickness (ideally ≥10 × δ)
Howthe DGT works
DGT facts➟ Generally only labile trace elements measured➟ Temperature-related effects are predictable➟ Diffusion coefficient independent of ionic
strength of receiving water (must be >1 mM)➟ Operating pH range of 5-10 for most elements➟ Not affected by hydrodynamic conditions➟ MDL for DGT after 1 day deployment is <4 pM
(concentration factor of ~300 times)➟ Analysis involves batch leaching (typically with
80 % recovery) followed by AAS or ICP-MS
Flow effects on Cd accumulation
C�concentrationDGT�diffusive gradient in thin-filmsASV�anodic stripping voltammetry
Cd accumulation with time &different gel thicknesses
Time (hours) 1/∆g (1/mm)
∆g = diffusive gel thickness
Mas
s C
d (n
g)
(Zhang & Davison, 1995)
Discrete sampling program➟ Manual quarterly sampling, typically near base
flow conditions (4 years)
➟ Automated storm sampling (4 years)
➟ Streamflow & water quality (T, C, pH, DO & turbidity) at 5-minute intervals (4 years)
➟ Estuarine grab samples collected & water quality measured in situ concurrently with DGT (8 months)
DGT study design
➟ Compare stream DGT results with data from discrete base-& storm-flow samples collected over 4 years
➟ Compare estuarine DGT results with discrete samples collected concurrently with DGT retrievals over 8 months
DGT processingStep 1 - DGT disassembly
Step 2 - Removal of resin gel
Step 3 - Resin gel leaching
Step 4 -ICP-MSanalysisof DGT leachate
Calculating mean concentration
where:Cw = mean metal concentration in waterM = mass diffused into DGT)g = diffusive hydrogel thickness +
membrane filter thicknessDT = diffusion coefficient at any temperaturet = deployment (exposure) timeA = area of DGT window
Stream results and why➟ Results of various methods for determining means
from discrete samples differed considerably➟ Rating curves were appropriate for upper but not
lower watershed (except for Pb)➟ DGT results generally comparable to, but less
than, grab sample means➟ DGTs measure the aquo ion, inorganic complexes,
and possibly small organic complexes & colloids➟ Grabs include larger colloids & organic complexes➟ No clear relation between flow & dissolved trace
element concentration
Estuary results & why➟ DGT deployments >2 weeks not recommended➟ Grab samples collected at different stages of tide &
under different streamflow & weather conditions➟ DGT results were significantly different (α = 0.05)
from concurrent grab results except for Co➟ DGT results were not consistently higher or lower
than results from concurrent grab samples➟ CuDGT > Cugrab & > chronic & acute HAR 11-54
standard (2.9 µg/L)➟ Need many more grab samples to accurately
characterize estuary
Conclusions:➟ To date this study is the longest deployment
of DGTs in diverse aquatic environments➟ DGTs preconcentrate dissolved trace
elements & remove matrix interference for ICP-MS
➟ DGTs are a simpler, faster, economical way to measure dissolved trace elements
➟ DGTs provide mean concentrations but they also can show long-term variability
Conclusions (continued):➟ Watershed DGT & sample mean trace
element concentrations were similar➟ DGT means, however, often were less than
means from discrete samples➟ DGTs measure aquo ions, inorganic
complexes, small organic complexes, & very small colloids
➟ DGTs do not measure trace elements in larger colloids or organic complexes & small particulates
Conclusions (continued):➟ Except for Co, DGT & concurrent estuary
samples were significantly different➟ Estuarine DGT results were not consistently
less or greater than discrete sample results➟ Dynamics & complexity of estuary requires
far more samples to characterize chemistry➟ DGTs can be deployed for up to 3 months
in relatively clean, freshwater systems
Conclusions (continued):➟ Biofouling limits DGT deployments
1-4 weeks in subtropical estuaries➟ Operational pH range for DGTs (5-10) is
normally not a problem➟ Ionic strength rarely < 1 mM (~0.2 % of the
time in the upper watershed during storms)➟ DGTs are viable method for measuring
dissolved trace elements in subtropical freshwater & estuarine environments
Revised Sampling Scheme➟ Multiple (>5) blank checks before deployment➟ Three replicate DGTs deployed at each site➟ Dilute leachate by no more than 4 times➟ Continue temperature recording with TidbiTs➟ Locate inexpensive conductivity recorder➟ Deploy mid-depth in deeper stream waters➟ Collect or locate OC & speciation data➟ Multiple depths & locations in estuary➟ Deploy short- and long-term DGTs in freshwater